答案：B

题目分析：句意：--你在哪里找到我们的化学老师的？--是在他做实验的实验室。根据句意这里是定语从句，先行词是the lab，定语从句中缺少的是地点状语，所以用关系副词where，这道题考生容易错选成that，因为是强调句，但是这里的强调句实际上被省略了，完整的句子是：It was in the lab where he was doing his experiment that I found our chemistry teacher. 所以选B。